Far-called, our navies melt away— on dune and headland sinks the fire— lo, all our pomp of yesterday is one with nineveh and tyre! what has happened to the “pomp of yesterday” in these lines from “recessional” by rudyard kipling

You can see in the excerpt that 'all our pomp of yesterday / Is one with Nineveh and Tyre!" This means that in order to answer your question, you need to know what Nineveh and Tyre are. Those are ancient cities which no longer exist and haven't existed for a very long time. Thus, what happened to the 'pomp of yesterday' is that it has disappeared like empires of old.